Many petroleum refineries utilize a delayed coking unit to process residual oils. In delayed coking, overhead vapors from the coking drum pass to a fractionator where various fractions are separated. One of the fractions is a gasoline boiling range stream. This stream, commonly referred to as coker gasoline or coker naphtha, is generally a relatively low octane stream, unsuitable for use as an automotive fuel without upgrading. With the limitations on octane improving additives imposed by environmental concerns, it has become desirable to process coker gasoline naphtha in an octane improving unit such as a catalytic reformer. Sulfur compounds in coker gasoline are detrimental to catalytic reforming catalysts, and accordingly it has become commonplace to treat coker gasoline in a catalytic desulfurizer prior to processing it in a catalytic reformer. These steps of delayed coking, desulfurization and reforming are well developed commercial processes which are widely used in the refining industry. Hydrodesulfurization (HDS) catalysts can be undesirably poisoned by silicon. Silicon can also poison reformer catalyst which is even more expensive.
In the delayed coking process, quantities of foam are often produced which are undesirable as the foam diminishes the efficiency of the delayed coking process. It is customary, as described in U.S. Pat. No. 3,700,587, to add silicone defoamers, for example polydimethylsiloxanes, to delayed cokers to control the undesirable foaming. Thus, one of the recurring concerns in the coker is contamination of the products with silicon species that can eventually poison hydrotreater catalyst and cause the expensive catalyst to be replaced prematurely. Since the silicone-based defoamer used in the coke drum is believed to be a major source of the silicon contamination, considerable effort has been expended to minimize or eliminate the use of silicone oil from the unit. The silicone oil acts both as a defoamer to knock down foam in the drum when the silicone oil is first injected and also acts as an antifoam agent to prevent the subsequent build-up of foam. Due to silicone oil's unique surface properties and thermal stability, there has not yet been a commercially viable non-silicone defoamer replacement.
There are two routes for the silicone to leave the coke drum and contaminate the products of the fractionator: 1) the original molecule of silicone can become physically entrained in the overhead gases, or 2) the original silicone molecule can break down into small enough pieces to distill over with the liquid products. The silicone that is typically used in cokers is of such high molecular weight that it will not distill at coker temperatures. However, L. Kremer in “Silicon Contamination of Coker Products”, AlChE 5th International Conference on Refinery Processes, Mar. 12, 2002, presents laboratory results on the thermal degradation of silicone oil. Breakdown products are primarily cyclic trimers and tetramers of polydimethylsiloxane, and have boiling points of 134° C. (273° F.) and 175° C. (347° F.), which explains why they distill in the coker products.
U.S. Pat. No. 4,176,047 describes that a gasoline boiling range hydrocarbon stream obtained by fractionation of overhead vapors from a delayed coker may be treated for removal of organic silicon compounds prior to being processed in a desulfurizer and catalytic reformer. The stream is treated by a bed of material such as alumina, activated alumina or spent alumina-based desulfurizer catalyst at elevated temperature to reduce the level of organic silicon compounds. The organic silicon compounds, if not removed, are detrimental to desulfurizer and reformer catalysts. The organic silicon compounds are conventionally added to a delayed coker to control foaming.
It would be desirable, however, to also provide a method and/or composition that would minimize the amount of silicon in the coker products in the first place.